Stars of certain metallicity and mass go on ``Blue Loop Excursions'', after
they ignite He in their core (see
Figure 1). Stars in the BL phase can be
several magnitudes brighter than when on the MS. The luminosity of a BL
star is fixed for a given age. Subsequent generations of BL stars do not
overlie each other as they do on the MS. The shape and mass at which these
``loops'' are seen in a CMD are a strong function of metallicity and age,
in the sense that the lower the metallicity of the galaxy the further back
in time an accurate SFH can easily be determined, which makes them
especially useful for low metallicity systems such as dI galaxies. (e.g.
Dohm-Palmer et
al. 1997a,
b,
1998;
Tolstoy et al. 1998).